Partners wanting Vasectomy

Erica
So I just want to vent. I'm about to have my first child with my fiancé. However he has a son from a previous relationship. Now he's saying he's done now since we're having a girl. I'm upset since it takes away the option if I want more children. I feel torn because I never imagined having only one baby. Now I feel like I'm being forced to. I don't want to get myself too upset because he hasn't actually done it yet....and we aren't married yet where I have more of a say.
Did anyone else deal with a partner that said they wanted a vasectomy and you were against it? How'd you handle it?
**Update to address some FAQ and comments:
• YES, we have discussed this together first. I'm only asking to connect with other women who may have or be experiencing the same thing. 
• YES, we discussed having children before I got pregnant. But we did NOT discuss him seriously considering a vasectomy where I would be limited to only one.
• NO, I honestly don't know how many other children I want yet. I just want the option there.